This invention relates to a process for molding a circuit component; and more particularly, to a two-shot thermoplastic molding process for manufacturing an electrical connector.
Electrical connectors and other similar electrical components often include electrical conductors embedded within an insulating housing to isolate the conductor from the surrounding environment. Embedding the conductor within a housing protects the conductor from damage, and also prevents the delivery of an electrical shock. Electrical isolation is particularly important when the connector is to be coupled to an implantable medical device such as a pacemaker or defibrillation system.
One way to form an electrical connector having conductors embedded therein is to mold a solid set-screw block using injection molding techniques. After the molding is completed, the surface of the set-screw block is formed to include channels. Wires or other types of connectors are pressed into the channels. Generally, each end of each wire is welded to some type of electrical contact. An insulating adhesive is then applied over the wires and channels. If the connector is to be used with an implantable medical device, a medical adhesive is often employed for this purpose. The adhesive is cured to form a protective, insulating layer that isolates the wires from external elements.
Although the afore-mentioned method is relatively straight-forward, it requires manual application of the adhesive. This introduces variables into the manufacturing process. If the adhesive is not properly dispensed, some portions of the conductor may become exposed. As a result, shorts may develop between adjacent conductors. Additionally, a conductor may come in contact with external elements, causing degradation and loss of conductive capabilities. Moreover, because a manual process is employed, the manufacturing mechanism is more time-consuming and expensive.
An alternative approach to the use of adhesives involves the positioning of one or more conductors within a mold in some predetermined orientation. An insulating plastic is then introduced into the mold to encapsulate the conductors. The plastic hardens to provide the necessary insulating layer around the conductors. While this process eliminates the variables associated with a manual step, it is nevertheless difficult to implement with other than a simple design. This is because the introduction of the plastic into the mold at high pressures generally causes the position of the conductors to shift. This may result in shorts between multiple conductors, or conversely, may result in loss of a desired electrical connection. While plastic injection systems of this nature generally include mechanisms to hold the conductors in place during the injection process, the process is more prone to failure than other methods because shifting of components may occur regardless of the efforts to prevent it. Additionally, a more complex tooling system is required to implement the process. Finally, the difficulty associated with maintaining isolation between multiple conductors places limits on the assembly dimensions. That is, an assembly cannot be made too small because shorts will occur between closely spaced conductors that shift during the mold injection process.
Yet another approach used to create connector assembly includes use of a two-step thermoset casting process. A first mold is used to receive a thermoset plastic material such as an epoxy. As is known in the art, a thermoset plastic hardens because of a chemical reaction occurring between the various components of the plastic material. After the curing process is complete, the first molded connector element is removed from the mold. Conductors are selectively positioned on the exterior of this first element. The first element is then positioned within a second mold and a thermoset material is selectively applied to the first element to encapsulate the conductors.
The two-step thermoset process provides a mechanism for embedding conductors within a connector in a more precise manner. This is because the first element holds the conductors in position while the second molding step is performed. However, because thermoset material requires a relatively long time to cure, the process is slow. The manufacture time is increased since two serial curing steps are required. Moreover, because the fmal products may not be removed from the molds until the curing is completed, many molds must be employed to increase output.
What is needed, therefore, is an improved mechanism for creating more complex connector structures using a faster production cycle.
The current invention provides an improved circuit assembly for use in an implantable medical device, and a method of making the assembly. The circuit assembly includes a core portion formed of a thermoplastic material using either an injection molding process or a machining process. This core portion is adapted to be fitted with at least one electrically conductive circuit component such as a connector member, a set-screw block, or a conductive jumper member. In one embodiment of the invention, the core portion includes multiple receptacles or other spaces that are adapted to be loaded with the various circuit components. Core portion may further be provided with groove and ridge members designed to position and retain conductive jumper members at predetermined locations at the surface of the core portion. Such conductive jumpers may be welded or soldered to a respective one of these circuit components to form electrical contacts between the jumpers and the respective circuit component.
After the electrically conductive circuit components are positioned in this manner with respect to the core portion, this core assembly is prepared for an overmolding process. This involves ensuring that certain portions of the core assembly will be protected from the flow of thermoplastic material during a subsequent overmold process. This process may include loading bushings into various connector members and/or set-screw blocks of the core element assembly.
When the core assembly has been prepared for the overmolding process, it is loaded into a second-shot mold. In one embodiment of the invention, the core assembly is aligned and retained within a cavity of the mold by utilizing slidable members that are provided by the mold, and that are adapted to engage the core assembly. Positioning of the core assembly within the mold cavity may further be accomplished using pegs that are adapted to engage various corresponding apertures of the core element assembly.
During the overmold process, a second-shot of thermoplastic material is injected into the mold. This thermoplastic material is heated to a temperature at, or above, the melting point of the material. This thermoplastic material is hot enough to create a bond between the core portion and the overmold material. To achieve this, the mass of the core element as compared to that of the overmold material is made as small as possible so that the heat energy from the mold is able to adequately heat the core portion. In one embodiment, the mass of the core portion is less than fifty percent of the mass of the overmold material, and preferably is less than thirty percent. Bonding may further be enhanced by providing ridges on the surface of the core portion that are melted during the overmold process and thereafter integrated with the overmold material. The bonding may also be facilitated by pre-heating the core portion prior to injecting the second shot of thermoplastic material into the mold.
In one embodiment of the invention, a hermetically-sealed connector assembly is provided for use with an implantable medical device (IMD) such as a cardioverter/defibrillator, a pacemaker, or any other type IMD that is adapted to coupled to medical electrical leads. Because the connector assembly is manufactured using thermoplastic materials, the manufacturing process may be completed in a much shorter amount of time than similar assemblies formed of thermoset materials. The connector assembly includes one or more connectors that are adapted to couple mechanically and/or electrically to the pin or ring connectors of a medical electrical lead. Such connector members may conform to various standards for medical electrical leads, such as IS-1 and DF-1 standards.
Other aspects and advantages of the current inventive circuit assembly system and method of making the circuit assembly system will be apparent from the drawings and accompanying detailed description of the invention embodiments.